1. Field of the Invention
This invention relates to a method for recovering solid molybdenum compounds. More particularly, this invention relates to a method for recovering solid molybdenum compounds from a hydrocarbon fraction having hydrocarbon soluble molybdenum compounds dissolved therein. Still more particularly, this invention relates to a method for the recovery of solid molybdenum compounds from a heavy liquid hydrocarbon fraction containing contaminating quantities of hydrocarbon soluble molybdenum compounds.
In its more particular aspects, this invention relates to an improvement in a process for the epoxidation of an olefin with an organic hydroperoxide in the presence of a soluble molybdenum peroxidation catalyst wherein the reaction mixture is resolved into fractions, including a substantially anhydrous heavy liquid fraction containing soluble molybdenum epoxidation catalyst and from which the solubilized molybdenum catalyst is precipitated by treating the heavy fraction with an aqueous solution of a molar excess of sodium borate relative to the gram atoms of molybdenum in the heavy fraction. The precipitated molybdenum can be separated from the heavy fraction by filtration in order to provide a filtrate containing less than 100 parts per million of molybdenum which can be safety incinerated without excessive fouling caused by plating out in the incinerator. Further, the 100 ppm or less limit on the molybdenum concentration in the filtrate tends to keep the corrosiveness of the molybdenum solution at acceptable levels. The molybdenum trioxide liberated by the burning of a filtrate containing 100 ppm molybdenum or less can be recovered by conventional bag filters or similar devices.
2. Prior Art
A process for the manufacture of substituted epoxides from alpha olefins such as propylene is disclosed in Kollar U.S. Pat. No. 3,351,653 which teaches that an organic hydroperoxide such as tertiary butyl hydroperoxide can be reacted with the olefin in the presence of a molybdenum catalyst. When the olefin is propylene and the hydroperoxide is tertiary butyl hydroperoxide, the principal reaction products are propylene oxide and tertiary alcohol. Kollar U.S. Pat. No. 3,351,653 is illustrative of the many patents that have issued relating to the reaction of hydroperoxides with olefins in the presence of molybdenum catalysts. Other references include, for example, U.S. Pat. No. 3,526,645, Wulff et al. U.S. Pat. No. 3,634,464, Harrod et al. U.S. Pat. No. 3,654,317, Sorgenti U.S. Pat. No. 3,666,777, Stein et al. U.S. Pat. No. 3,849,451, etc.
The molybdenum compound that is used to catalyze the epoxidation reaction should be a molybdenum compound that is soluble in the reaction medium. A wide variety of molybdenum compounds have been proposed for this purpose, including the molybdenum compounds disclosed in Kollar U.S. Pat. No. 3,351,653. Thus, U.S. Pat. No. 4,434,975 discloses molybdenum catalysts prepared from saturated alcohols or glycols. See also Cavitt U.S. Pat. Nos. 3,784,482 and 3,787,329.
Sorgenti U.S. Pat. No. 3,573,226 discloses a molybdenum-containing epoxidation catalyst solution prepared by heating molybdenum powder with a stream of unreacted tertiary butyl hydroperoxide in polyhydric compounds and Lines et al. U.S. Pat. No. 3,953,362 discloses molybdenum epoxidation catalysts prepared by reacting an oxygen-containing molybdenum compound with hydrogen peroxide and an amine.
The reaction product that is formed will contain not only unreacted organic hydroperoxide and unreacted olefin, the desired epoxide reaction product and the alcohol corresponding to the hydroperoxide feedstock, but will also contain impurities formed during the course of the reaction or introduced into the reaction mixture with the feed components. Among the impurities that are present are acidic oxygen-containing contaminants and by-products including acetone, formic acid, acetic acid, isobutyric acid, the methyl esters of such acids, etc.
After the desired reaction products have been recovered from the reaction mixture, the remaining components of the reaction mixture must be processed for recycle or disposal. The processing of the remaining components presents a problem insofar as the molybdenum catalyst is concerned, because it will normally be present in the reaction mixture in a very small amount, usually less than 1000 ppm.
Normally, after the separation of the desired reaction products the molybdenum will be present in a heavy organic stream which may be a distillate bottoms fraction that remains after unreacted propylene, propylene oxide and olefin oxide epoxide and product alcohol have been recovered.
Levine U.S. Pat. No. 3,819,663 discloses a method for recovering the molybdenum from the heavy organic stream by evaporating most of the liquid components from the stream in a wiped film evaporator in order to provide a residue which Levine states can be recycled back to the reaction mixture.
Such concentration usually leads to a concentration of acids and polyglycols and the presence of these materials in the recycle catalyst solution leads to lower epoxide selectivities and, therefore, it is often not feasible to recycle the molybdenum to the reaction zone and it must be recovered for disposal (e.g., to a company that reclaims molybdenum from solids).
U.S. Pat. No. 3,463,604 discloses the use of ammonium phosphate and related compounds to precipitate molybdenum.
U.S. Pat. No. 3,887,361 discloses heating the effluent in the presence of tertiary butyl alcohol to form a solid precipitate.
U.S. Pat. No. 4,317,802 discloses a related process wherein the effluent is heated with water below the saturation point to precipitate aluminum. U.S. Pat. Nos. 4,485,074 and 4,547,345 disclose processes wherein the epoxidation effluent is heated with an inert gas and with water in order to induce precipitation. A related process is disclosed in European patent application No. 56,740.
Another patent, U.S. Pat. No. 4,317,801 discloses the use of aqueous hydrogen sulfide to precipitate the molybdenum.
However, if a molybdenum recovery process is to be satisfactory, the precipitation must be so complete that the heavy fraction from which the molybdenum is removed can be burned, for example, without fouling the incinerator or causing excessive corrosion in the incinerator.